During the first year of life, infants spend most of their time in the sleeping state. Assessment of sleep during infancy presents an opportunity to study the impact of sleep on the maturation of the central nervous system (CNS), overall functioning, and future cognitive, psychomotor, and temperament development. Sleep is essential to human life and involves both physiologic and behavioral processes. During the first year of life, infants spend most of their time in the sleeping state. Sleep not simply as a resting state, but a state that involves intense brain activity.[1]
The first year of life is a time of substantial change in the development of both the human brain and sleep. The relationship between the two is vital, as the control of sleep and the sleep-wake cycle are regulated by the CNS.[2]

Initially, the sleep of infants is characterized by two different types of sleep: active sleep and quiet sleep[3].

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In the first weeks of life, they spend about half their sleeping time in active sleep. The EEG profile is similar to that seen in adult REM sleep. However, the appearance of rapid eye movements is not associated with a complete motor paralysis as it is in adults, because of the immaturity of the cerebral inhibitory mechanisms that control transmission of nerve impulses to the skeletal muscles. So small body movements, may occur, particularly in the facial muscles, fingers and toes.

Active sleep decreases gradually during the first year, leveling off at about 25–30% at 1 year of age. At this age body movements during REM sleep gradually disappear, and muscle atonia is observed as in adults.

Quiet sleep of infants is simiar to NREM sleep in adults, but well-defined delta waves and sleep spindles cannot be detected during the first weeks of life because electrical brain activity is still irregular and disorganized at this time. REM sleep depends on the activity of the reticular neurons in the pons, which are already relatively mature at birth; however, it is only when the cells in the cortex become extensively interconnected and when their connections to and from the thalamus are developed that the typical EEG patterns of slow wave sleep (SWS) are observed and distinct sleep Stages 2, 3, and 4 develop from quiet sleep[4].

The cycle of NREM-REM sleep is present from birth, but the rate of switching in the newborn occurs at 50–60 minutes intervals. A more mature sleep cycle is gradually acquired,
and the adult EEG patterns of NREM sleep stages emerge during the first 2–6 months of life. Another major difference between infant and adult sleep is that in newborns the
transition from wake to sleep is often accomplished through active sleep. The declining levels of REM sleep during the first year of life are in part the result of an increase in the noradrenergic and serotonergic inhibitory control of REM and in part the result of neurons becoming less sensitive to acetylcholine, a neurotransmitter that enhances REM sleep [5].

When brain structure and function achieve a level that can support the high-voltage characteristic of SWS, then Stages 3 and 4 become quite prominent in young children. In addition,
their SWS is much deeper than the SWS of older adults, with a very high arousal threshold. The amount of SWS achieved per night is maximal in young children but decreases markedly with age; it decreases by nearly 40% during the second decade of life [6]

The long sustained sleep period (LSP) is the period of time that a child sleeps without awaking. The length of this period increases dramatically between the first and second months. Between the ages of three and twenty-one months, LSP plateaus, increasing on average only about 30 minutes.[7] In contrast, a child’s longest self-regulated sleep period (LSRSP) is the period of time where a child, without sleep problems, is able to self-initiate sleep without parental intervention upon waking.[7] This self-regulation, also called ‘’’self-soothing’’’, allows the child to consistently use these skills during the nocturnal period. LSRSP dramatically increases in length over the first 4 months, plateaus, and then steadily increases at 9 months. By about 6 months, most infants can sleep 8 hours or more at night uninterrupted or without parental intervention upon awaking.[7]

In terms of actual numbers, an infant from one to three months of age may sleep sixteen to eighteen hours a day in periods that last from three to four hours. By three months the period of sleep lengthens to about four or five hours, with a decrease in the total sleep time to about fourteen or fifteen hours. At three months, they also start to sleep when it is dark and wake when it is light. By 4 months there are 2 distinct napping periods, mid-morning and late afternoon. By 6 months the longest LSP is 6 hours and occurs during the night. There are two 3-or-more hour naps with a total average sleep time of fourteen hours.[8]

Though sleep is a primarily biological process, it can be treated as a behavior. This means that it can be altered and managed through practice and can be learned by the child. Healthy sleep habits can be established during the first four months to lay a foundation for healthy sleep. These habits typically include sleeping in a crib (instead of a car seat, stroller, or swing), being put down to sleep drowsy but awake, and avoiding negative sleep associations, such as nursing to sleep or using a pacifier to fall asleep, which may be hard to break in the future.[8]

Every child is different and each child’s sleep becomes regular at different ages within a particular range. In the first few months of life, each time the baby is laid down for bed and each time he or she awakens is an opportunity for the infant to learn sleep self-initiation and to fall asleep without excessive external help from their caregiver.
Experts say that the ideal bedtime for an infant falls between 6 pm and 8 pm, with the ideal wake-up time falling between 6 am and 7 am. At four months of age, infants typically take hour naps two to three times a day, with the third nap dropped by about 9 months. By 1 year of age, the amount of sleep that most infants get nightly approximates to that of adults.[8]